The present invention relates generally to apparatus and methods for coating the interior surface of a pipeline and, in a preferred embodiment thereof, more particularly provides methods and associated apparatus for the in situ interior application, in extended, deteriorated underground pipelines, of a multi-layer protective and rehabilitative coating. The coating may be formed in a total thickness sufficient to not only seal leaks in the pipeline and inhibit further interior corrosion thereof, but to provide a very substantial amount of structural reinforcement to the deteriorated pipeline as well. Importantly, absent large holes or breakages in the pipeline, the interior coating technique of the present invention may be employed without excavation except at the opposite ends of the pipeline.
As is well known, several miles of corroded underground metal pipeline typically present a near monumental problem to the pipeline owner or operator. Total replacement of the pipeline is very often out of the question from both economic and downtime standpoints. A logical alternative is to laboriously locate the most seriously deteriorated sections of the pipeline, excavate at their locations, and patch them or replace them with new pipe. This "patchwork" solution, as might be imagined, presents an ongoing project which merely defers portions of the necessary repair/replacement cost and, at best, is far from satisfactory. After the worst pipe sections have been repaired or replaced it is often only a short time before the second worst sections begin to leak.
Thus, for many years, an in situ method of internal pipeline repair has been rather eagerly sought, and various proposals have been made as to how best to avoid total or "patchwork" replacement of deteriorated underground pipelines by repairing them from the inside along their entire in-place lengths.
One in situ method proposed entails the cable-pulling of a flexible polyethelene liner pipe into and through the deteriorated metal pipeline to form therein a continuous corrosion barrier, the pulled-in liner pipe being compressively "set" to the interior surface of the metal pipeline. To carry out this pipe lining process it is necessary to excavate at spaced intervals along the buried metal pipeline and remove sections thereof at the various excavation locations. Special reconnection flanges must then be secured to each pair of facing pipeline ends. With the now sectioned pipeline prepared for liner installation, appropriate lengths of the liner pipe are cablepulled and set into place within their associated pipeline sections. The liner pipe sections must then be joined, the pipeline sections reconnected, and the excavation holes refilled. Particularly where the deteriorated pipeline extends for several miles, this is a rather costly and time-consuming process which can result in extended pipeline downtime.
For example, as a practical matter the maximum installation length of each pipe liner section is from about 2,500 feet to about 5,000 feet. Thus, in a twenty mile underground pipe line approximately twenty to forty separate excavations must be made, with generally the same number of separate pipeline cuts, liner pulls, pipeline rejoining steps and excavation refills being required.
Another conventional in situ pipeline repair technique entails the longitudinal pulling through each previously cut apart section of the pipeline a length of reversible pipe lining material having, on its resulting outer side surface, an adhesive material which bonds the flexible liner section to the interior surface of its pipeline section. Like the liner pipe method described above, this method requires considerable excavation, pipeline cutting and reconnection, pipeline downtime, and considerable expense.
As an alternative to these liner pipe and eversible flexible liner techniques, various internal pipeline coating methods have been proposed. For example, self-propelled or cable-pulled spray coating machines of various types have been used to internally coat pipelines with a corrosion inhibiting substance. These, methods, like their liner counterparts, involve considerable excavation and the cutting of the corroded pipeline into a series of end-to-end sections whose lengths are limited by the maximum usable cable length. Additionally, of course, the pipeline sections must be suitably rejoined, and the various excavations refilled.
Finally, as described in U.S. Pat. No. 3,108,012 to Curtis, an in situ interior coating process for deteriorated underground pipelines has been proposed in which spaced apart pipeline "pigs" are air driven through various sections of the pipeline to perform cleaning and protective coating operations therein. During the cleaning phase of the operation, a suitable cleaning agent is disposed between the pigs, and during the coating operation a solvent-based coating liquid, akin to a "paint", is disposed therebetween.
While the Curtis method is considerably simpler than the solid lining and spray coating techniques, it has a variety of problems, limitations and disadvantages. For example, the liquid, solvent-based coating material may only be applied in a relatively thin layer, on the order of 0.005" to about 0.006", if satisfactory drying (i.e., solvent removal) is to be achieved. If a thicker layer is applied, incomplete drying occurs which often results in portions of the coating being washed away when the pipeline is returned to service.
Maintenance of this maximum coating thickness is rendered nearly impossible due to the propensity of the solventbased coating to "run" prior to the drying thereof. This tends to cause the coating to "puddle" on the bottom of the pipeline, and to drip into the corrosion pits therein (which may often be 0.200" deep). Thus, the coating on the upper interior pipeline is often thinner than desired and the coating thickness on the bottom of the pipeline can be far greater than that which can be satisfactorily dried and cured. Accordingly, when the pipeline is returned to service, large areas of uncured coating are often simply washed away--particularly from the corrosion pits in the of the pipeline. This now exposed pitted area is then subjected to a concentrated corrosive action of the fluid traversing the pipeline when it is returned to service.
Moreover, the Curtis process is basically an interior pipe "painting" scheme. The resulting thin coating on the pipe interior does not add any appreciable measure of structural reinforcement (often sorely needed) to the corroded pipeline.
In view of the foregoing, it is accordingly an object of the present invention to provide methods and associated apparatus for the in situ interior protective coating of extended pipelines which eliminates or substantially minimizes the above-mentioned and other problems, limitations and disadvantages typically associated with conventional techniques for providing corrosion repair and rehabilitation for deteriorated pipelines.